In recent years regulations on particulate matter emitted by internal combustion engines (mainly consisting of solid carbon particulates, sulfate and other sulfur containing particulates and liquid and solid hydrocarbon particulates) have been directed towards becoming more rigid due to their harmful impact on human body. Therefore to reduce the amount of emissions has been attempted in various fields. The field of the catalysts for purification of exhaust gas is no exception. Until now the methods for reducing particulate matter can be divided mainly into two fields: Catalysts with the function to trap particulate matter and oxidation catalysts with open flow substrates.
Under them catalyst technologies with trapping function mainly use substrates made of cordierite with wall flow, further to improve the efficiency of trapping particulate matter substrates made of silicon carbide are proposed. This method is intended to reduce exhausted particulate matter by trapping with a filter. This method is especially efficient if the particulate matter in the exhaust gas has a high ratio of dry soot. However, if the particulate matter is accumulated over a certain level back pressure is rising and a load can be inflicted on the engine even with the possibility leading to a complete engine stop. Especially in the case of the exhaust gas of an automobile to trap and burn particulate matter continuously throughout the whole range of engine operation is difficult because under various operation conditions of the engine the exhaust gas temperature is differing widely. Therefore, following methods are presently thought of to deal with above mentioned problem: 1) Trapping particulate matters in the low temperature area and burning them off in the higher temperature area. 2) Heat treatment methods to raise the temperature of the exhaust gas for regeneration of the catalyst by e.g. engine control, bypass and electrical heating. However, installing such heat treatment systems into automobiles is not quite realistic from the viewpoint of cost and space. Further a sudden combustion of trapped particulate matter in the catalyst leads to a dramatic rise of temperature therein leading to aging related problems such as melting of the filter.
On the other hand, heretofore oxidation catalysts have been the most reliable technologies and are widely in practical use. Oxidation catalysts are generally composed of an open flow honeycomb substrate coated with a catalytic active material. Their function is to reduce the soluble organic fraction (hereinafter referred to as SOF) from the particulate matter by absorption and subsequent combustion and decomposition. Hereby the amount of particulate matter is reduced, however, if the exhaust gas from the engine contains SOF in a low ratio, the particulate matter conversion is proportionally lower. Additionally in the case of high sulfur concentration in the fuel sulfur discharges at higher temperatures leading to higher particulate matter emissions which is another reason for concern.
An additional problem is that the catalysts function to trap and combust SOOT a considerable part of particulate matter is low resulting in a low conversion rate for particulate matter. Even at such low temperatures as such of an exhaust gas from a Diesel engine, which are remarkably lower than those of a gasoline engine (preferably lower than 350° C.), combustion of inflammable carbon particulates is requested from a good catalyst.
However, in heretofore proposed oxidation catalysts the part of the three dimensional structure which comes into contact with the gas is coated with a layer in which the catalytic active components are present as very fine particles. Therefore the contact efficiency between SOOT and the catalyst is very low and the catalytically active material is unable to bring up a sufficient combustion performance.
Therefore in recent years several methods have been proposed to raise the trapping efficiency. For example an attempt to higher the SOOT trapping efficiency of SOOT is by usage of an open flow substrate in which inorganic fibers are attached to the channel walls (the official gazette of JP-A-59-14282) or by the usage of an open flow honeycomb substrate with randomly arranged protuberances in large number on the channel walls (the official gazette of JP-A-57-99314). Another attempt to improve trapping SOOT and contact efficiency between it and the catalytically active component leading to higher SOOT combustion performance is by depositing a catalytically active component in form of protuberances on the gas inlet side of a diaphragm type gas filter (the official gazette of JP-A-7-24740). Also open flow or plugged type substrates that are able to raise trapping efficiency of SOOT were proposed. To generate protuberances on the channel walls coarse ceramic particles are attached or the surface of the channel walls are foamed (the official gazette of JP-A-58-14921).
However, the invention disclosed here is using an open flow honeycomb substrate with an average pore diameter of 10 to 40 μm in the channel walls. The catalytically active material and/or heat resistant inorganic substance is coated in form of protuberances onto the channel walls of above mentioned substrate without blocking its pores. By this trapping of SOOT and contact efficiency are improved leading to a raise in combustion performance of the catalyst. Such an invention has never been disclosed to date.
On the other hand in the official gazette of JP-A-10-151348 an open flow type oxidation catalyst containing cerium oxide and zirconium oxide is disclosed. The substrate used for this catalyst contains at least cerium oxide or zirconium, or both. By coating at least one metal oxide from the group of copper, iron and manganese onto this substrate, particulate matter combustion with high efficiency is possible. Further a technology using a coating of a metal of the platinum group and other catalyst components onto a filter type substrate has been proposed. Depending on the oxidation capacity of the catalytic material used, accumulated particulate matter in the filter can be removed by oxidation and combustion.
Further a catalyst can be thought of in which the particulates are combusted immediately when they come into contact with the catalytically active metal. If such a catalyst can be realized the above mentioned regeneration methods become obsolete because in such a catalyst a continuous regeneration takes place. However, in case high amounts of particulate matter is emitted from the engine, especially in the low temperature range the oxidation performance of the catalyst is not sufficient for a complete combustion and particulate matter is accumulating in the catalyst. In this case oxygen can not access with particulate matter blocked catalytically active metal sites and at the same time contact efficiency of newly incoming particulate matter with the catalyst is declining and therefore oxidation of particulate matter by the catalyst becomes increasingly difficult. Additionally at higher temperatures the accumulating part of hardly inflammable dry SOOT is high which makes a regeneration treatment of the catalyst necessary.
With the above mentioned problems in mind the purpose of the invention disclosed here is to provide an oxidation catalyst not using a filter but an open flow type substrate that is able to remove particulate matter by combustion from the free flowing exhaust gas.
Consequently, the object of the invention disclosed here is to offer a new catalyst for purifying exhaust gas from an internal combustion engine, the production thereof and a method to purify the exhaust gas from an internal combustion engine.
Another purpose of the invention disclosed here is to provide an exhaust gas purifying catalyst that is reducing harmful components especially particulate matter from the exhaust gas by trapping, combustion and decomposition. Also the production thereof and a method for purifying the exhaust gas from an internal combustion engine is described.
The objects mentioned above are accomplished by the following items (1)-(15).
(1) A catalyst for the purification of an exhaust gas of an internal combustion engine, formed by using an open flow honeycomb provided in the channel walls of cellular monolithic substrates with pores having an average diameter in the range of 10-40 μm.
(2) A catalyst according to item (1), wherein the channel walls in an open flow honeycomb substrate are coated with a catalytically active component.
(3) A catalyst according to item (1), wherein the catalytically active component is applied to the channel walls in the open flow honeycomb substrate by wash coating.
(4) A catalyst according to any of items (1)-(3) wherein the honeycomb has a rib thickness in the range of 0.05 mm-0.50 mm and a porosity in the range of 60-90%.
(5) A catalyst according to any one of items (1)-(4), wherein the amount of the catalytically active component coated to the substrate is in the range of 5-200 g/liter and the average diameter of pores in the channel walls in the catalyst after wash-coating of the catalytically active component is in the range of 10-40 μm.
(6) A catalyst for the purification of the exhaust gas of an internal combustion engine, having attached to an open flow honeycomb substrate a film comprising coarse granular protuberances of a catalytically active component and/or a heat-resistant substance and preventing pores in the channel walls in the substrate from being blocked and having formed in the substrate such pores as possess an average diameter in the range of 10-40 μm.
(7) A catalyst according to item (6), wherein the protuberances on the channel walls are formed of coarse granules containing granules exceeding 40 μm in diameter at a ratio of not less than 80% by weight and granules exceeding 300 μm in diameter at a ratio of not more than 5% by weight.
(8) A catalyst according to any one of items (1)-(7), wherein the catalytically active component contains at least one element selected from the class consisting of alkali metals, alkaline earth metals, and rare earth metals of Groups IIIB-VB and Period 3 and Groups IIIA-VIIA, VIII, IB-IVB and Periods 4, 5, and 6.
(9) A catalyst according to either of items (6) and (7), wherein the heat-resistant inorganic substance used in forming the coarse granular attached film contains at least one member selected from the class consisting of activated alumina, silica, titania, zirconia, silica-alumina, alumina-zirconia, alumina-titania, silica-titania, silica-zirconia, titania-zirconia, and zeolites.
(10) A method for the production of a catalyst for the purification of the exhaust gas of an internal combustion engine set forth in any one of items (6)-(9), characterized by mixing a coarse granular substance together with at least one dispersing agent selected from the group consisting of alumina sol, titania sol, zirconia sol, silica sol, soluble boehmite, and soluble organic macromolecular compounds into an aqueous slurry and applying the aqueous slurry and a catalytically active component by wash coating to an open flow honeycomb containing pores of an average diameter in the range of 10-40 μm in the channel walls thereof.
(11) A method according to item (10), wherein the catalytically active component is applied by wash coating after the aqueous slurry has been applied by wash coating.
(12) A method according to item (10), wherein the catalytically active component is applied by wash coating simultaneously with the aqueous slurry.
(13) A method for the purification of the exhaust gas of an internal combustion engine, characterized by passing the exhaust gas of the internal combustion engine through a catalyst set forth in any one of items (1)-(12).
(14) A method for the purification of the exhaust gas of an internal combustion engine, characterized by having the catalyst for the purification of an exhaust gas of an internal combustion engine set forth in any one of items (1)-(13) disposed on the upstream side or downstream side of an oxidizing catalyst relative to the flow of the exhaust gas.
(15) A method for the purification of the exhaust gas of an internal combustion engine, characterized by having the catalyst for the purification of an exhaust-gas of an internal combustion engine set forth in any one of items (1)-(13) disposed on the upstream side or downstream side of an NOx reducing catalyst relative to the flow of the exhaust gas.